Neurological disorders afflict a substantial number of individuals and present an increasing economic challenge to health care systems since little is known regarding their causes, their diagnosis is often subjective, and many lack effective treatment. In general, brain activity is ultimately determined by the capacity of neurons to communicate at synapses. Specific neurotransmitter chemicals are packaged in presynaptic neurons into synaptic vesicles which fuse with the presynaptic membrane to release quanta of the neurotransmitter chemical that traverse the synaptic cleft to activate the corresponding receptor type resident in the post-synaptic membrane. Among these receptor types are the neuronal glutamate receptors (GluR's), δ-aminobutyric acid receptors (GABAR's), nicotinic acetylcholine receptors, serotonin receptors, dopamine receptors, and the like. Many neurological disorders are a result of improper conduction of electrical currents through synapses in various brain tissues. In epilepsy errant currents, hypothesized to be associated with improper function of synapses, cause various levels of seizures. Likewise, in several psychiatric diseases, movement disorders and neurodegenerative diseases the conduction currents become abberant, disorganized or reduced, thereby causing the disease condition. Accordingly, defects in synaptic vesicle functions will have an adverse effect upon neurotransmission in general and control of neurotransmitter release in particular.
Seizures, including epileptic seizures, result from a focal or generalized disturbance of cortical function, which may be due to various cerebral or systemic disorders, including, for example, cerebral edema, cerebral hypoxia, cerebral trauma, central nervous system (CNS) infections, congenital or developmental brain defects, expanding brain lesions, hyperpyrexia, metabolic disturbances and the use of convulsive or toxic drugs. It is only when seizures recur at sporadic intervals and over the course of years (or indefinitely) that epilepsy is diagnosed.
Epilepsy is classified etiologically as symptomatic or idiopathic with seizure manifestations that fall into three general categories: 1) generalized tonic-clonic, 2) absence or petiti mal, and 3) complex partial. Symptomatic classification indicates that a probable cause exists and a specific course of therapy to eliminate that cause may be tried, whereas idiopathic indicates that no obvious cause can be found and may be linked to unexplained genetic factors. Of the seizure categories, most persons have only one type of seizure, while about 30% have two or more types.
The risk of developing epilepsy is 1% from birth to age 20 yr. and 3% at age 75 yr. Idiopathic epilepsy generally begins between ages 2 and 14. Seizures before age 2 are usually caused by developmental defects, birth injuries, or a metabolic disease. Those beginning after age 25 may be secondary to cerebral trauma, tumors, or cerebrovascular disease, but 50% are of unknown etiology.
Due to the many interrelationships that exist between the nervous and endocrine systems, defects in synaptic vesicle function can also impact on endocrinological function. For instance, at least two glands secrete their hormones only in response to appropriate neurotransmitter release—the adrenal medulla and the posterior pituitary gland. Upon secretion, hormones are transported in the blood to cause physiologic actions at distant target tissues in the body. Obviously, endocrinopathies involving either hyper- or hyposecretion of hormones have pathological consequences. Examplary of these consequences are giantism and dwarfism, due to hyper- or hyposecretion of growth hormone, respectfully.
Levetiracetam
Levetiracetam (LEV; ucb L059; (S)-α-ethyl-oxo-pyrrolidine acetamide), the (S)-enantiomer of the ethyl analog of piracetam, was synthesized during a follow-up chemical program aimed at identifying a second-generation nootropic drug. In vivo results have demonstrated an unexpected potent ability of LEV to suppress seizures in the audiogenic-susceptible mouse, whereas piracetam was only weakly active. Although LEV is a molecule unrelated to established antiepileptic drugs (Margineanu et al., in Antiepileptic Drugs: 5th Edition, pp. 419–427. Lippincott, Philadelphia (2002)), extensive clinical trials have proven that adjunctive therapy with LEV (KEPPRA, UCB, S. A., Braine-l'Allend, Belgium) is both effective and well tolerated in controlling refractory partial seizures in adults.
Binding assays with LEV, performed on crude rat brain membranes, reveal the existence of a reversible, saturable and stereoselective specific binding site. Results obtained in rat hippocampal membranes suggest that LEV labels a single class of binding sites with modest affinity and with a high binding capacity. This binding site is identified as the Levetiracetam Binding Site (LBS). Similar results have been obtained in other brain regions (cortex, cerebellum and striatum), ucb L060, the (R)-enantiomer of levetiracetam, displays about 1000 times less affinity for these sites. The binding of LEV appears to be confined to membranes in the central nervous system since radiolabel studies could detect no specific binding in a range of peripheral tissues including heart, kidneys, spleen, pancreas, adrenals, lungs and liver. However, this could be due to a low density of LBS in these tissues compared to the central nervous system and indeed specific binding does occur in PC12 cells, a peripherally derived adrenal cell line. The most commonly used antiepileptic drugs carbamazepine, phenytoin, valproate, phenobarbital and clonazepam, as well as the convulsant agent t-butylbicyclophosphorothionate (TBPS), picrotoxin and bicuculline do not displace LEV binding. However, ethosuximide, pentobarbital, pentylenetetrazole and bemegride competed with LEV with pKi values comparable to active drug concentrations observed in vivo. Structurally related compounds, including piracetam and aniracetam, also displaced LEV binding. The levetiracetam analogues were also tested for their anticonvulsant activity in the audiogenic mouse model of epilepsy. A very good correlation (r2=0.84) was observed between the affinity and the anticonvulsant activity (Noyer et al., Euro. J. Pharmacol. 286:137–146. (1995)). This high degree of correlation is strong support for a causative relationship between LBS binding and anticonvulsant activity of this class of compounds. Accordingly, binding of levetiracetam analogues to LBS is expected to result in modification of the function of the protein component(s) of the LBS in brain, leading to the desired therapeutic outcome of anticonvulsant activity.
The Synaptic Vesicle Protein 2 Family
The Synaptic Vesicle Protein 2 (SV2) family of synaptic vesicle proteins was first identified with a monoclonal antibody prepared against cholinergic vesicles from the electric organ of the marine ray D. ommata (Buckley et al., J. Cell Biol. 100:1284–1294. (1985)). Cloning of the individual family members labeled by the antibody resulted in the identification of three different isoforms, SV2A (Bajjalieh et al., Science. 257:1271–1273. (1992)), SV2B (Feany et al., Cell. 70(5):861–867. 1992) and SV2C (Janz and Sudhof, Neuroscience 94(4): 1279–1290. (1999)), all of which react with the original antibody. The overall homology between the three rat isoforms is approximately 60%, with SV2A and SV2C being more similar to each other than SV2B (Janz and Sudhof, Neuroscience 94(4): 1279–1290. (1999)).
The SV2 proteins are integral membrane proteins and have significant but low-level homology (20–30%) to the twelve transmembrane family of bacterial and fungal transporter proteins that transport sugar, citrate, and xenobiotics (Bajjalieh et al., Science. 257:1271–1273. (1992)). As putative members of the 12 TM superfamily, SV2 proteins display several unique features. They have relatively short free N- and C- termini and short loops connecting the Tm segments. Two notable exceptions, however, are the long cytoplasmic loop between transmembrane regions 6 and 7 and the intravesicular loop between transmembrane regions 7 and 8 (which contains 3 N-glycosylation sites). No close homologs of the SV2 proteins have yet been discovered in yeast or invertebrates, although a distantly related synaptic vesicle protein known as SVOP does have homologs in Drosophila and C. elegans (Janz et al., J. Neurosci. 18(22):9269–9281. (1998)).
As a family, SV2 proteins are widely distributed in the brain and in endocrine cells. The three isoforms overlap significantly in their distribution, and can be found co-expressed in the same neuron, and even on the same synaptic vesicle. One isoform or another of the SV2 proteins seems to be present on all synaptic vesicles, and they are probably not limited to neurons that contain any specific neurotransmitters, although one study reports that cholinergic vesicles may not contain SV2 (Blumberg et al., J. Neurochem. 58(3):801–810 (1992)). SV2 proteins are therefore one of the most common proteins of synaptic vesicles, and have been implicated in the control of calcium-mediated exocytosis of synaptic vesicles. SV2 proteins have also been shown to be expressed in endocrine cells and, along with the additional synaptic vesicle membrane integral proteins p38 and p65, has been demonstrated to be present in endocrine dense core granule membranes (Lowe et al., J. Cell. Biol. 106(1):51–59(1988). SV2A, the most common SV2 isoform, is expressed ubiquitously throughout the brain and is present as well in secretory granules of endocrine cells. SV2B, while broadly distributed in the brain, is undetected in several brain structures, including the dentate gyrus of the hippocampus, the globus pallidus, reticular nuclei of the thalamus, and the reticular part of the substantia nigra (Bajjalich et al., 1994). By contrast, SV2C has quite a limited distribution and is found primarily the phylogenetically old regions such as the pallidum, the substantia nigra, the midbrain, the brainstem and the olfactory bulb. It is undetectable in the cerebral cortex and the hippocampus, and found at low levels in the cerebellar cortex (Janz and Sudhof, Neuroscience 94(4): 1279–1290. (1999)).
In addition to the SV2 protein, the synapse contains other unique regulatory proteins such as synapsin, synaptotagmin and CAPS, which may mediate vesicle fusion or budding. SV2A may be a Ca2+ regulatory protein essential for the formation of pre-fusion complexes called SNARE complexes (Xu et al. Cell 99(7):713–722 (1999)), which include the synaptic vesicle-associated VAMP/synaptobrevin and the plasma membrane proteins syntaxin and SNAP-25. Upon Ca2+ accumulation in the synapse the binding of synaptotagmin to SV2A is inhibited and the dimerization of two synaptotagmin Ca2+ binding domains is stimulated (Bajjalieh, Curr. Opin. Neurobiol. 9(3):321–328. (1999)). This dimerization may play a role in organizing the SNARE complex and promoting vesicle fusion, as at low Ca2+ concentrations, SV2A remains bound to synaptotagmin and fusion will not occur.
The affinity of SV2A for synaptotagmin is regulated by the phosphorylation of the amino terminus of SV2 (Pyle et al., J. Biol. Chem. 275(22):17195–17200. (2000)). The possibility that SV2 proteins play a role in either Ca2+ transport, or regulation in the synaptic vesicle has been supported by studies of SV2A and SV2B knockout animals (Janz et al., Neuron 24:1003–1016. (1999)). An alternative hypothesis is that the SV2 proteins, while derived from transport proteins, now serve a different function in the vesicle, whether a structural role or a role in regulation of vesicle fusion or recycling and the exocytotic release of their contents (Janz and Sudhof, Neuroscience 94(4): 1279–1290. (1999)).
There have been two reports of SV2 protein knockout mice: one that examines only SV2A knockouts (Crowder et al., Proc. Nat. Acad. Sci. USA 96(26):15268–15273. (1999)) and the other which looks at both SV2A and SV2B knockout animals, as well as the SV2A/SV2B double knockout (Janz et al., Neuron 24:1003–1016. (1999)).
Animals homozygous for SV2A gene disruption appear normal at birth, but fail to grow, experience severe seizures, and die within the first few weeks postnatal. SV2A homozygous knockout mice experience seizures that are longer lasting, stronger, and more debilitating than any other mouse strain (Janz et al., Neuron 24:1003–1016. (1999)). Despite the appearance of postnatal seizures, all SV2A knockout animals have completely normal gross brain morphology, including normal levels of the tested synaptic proteins. Furthermore, the hippocampal neuronal cultures from both SV2A and SV2A/SV2B double knockout mice formed synapses that were ultrastructurally normal, and had unchanged size, number and location of synaptic vesicles (Janz et al., Neuron 24:1003–1016. (1999); Crowder et al., Proc. Nat. Acad. Sci. USA 96(26):15268–15273. (1999)). It is interesting to note that, unlike the frequently observed seizures caused by structural and developmental abnormalities easily detected in many other type of knockouts, the SV2A knockout mice show a strong seizure phenotype with no associated macro or micro scale abnormalities of the brain or synapse. This observation suggests a direct and specific role for SV2A and the observed phenotype. As another marker of brain function, studies of synaptic transmission in primary neuronal cultures from SV2A, SV2B, and SV2A/SV2B knockout mice indicate that the sizes and frequencies of sIPSCs and of spontaneous excitatory postsynaptic currents (sEPSCs), are normal. Electrical stimulation induced robust EPSCs and IPSCs in the cultured neurons from all genotypes.
In contrast to SV2A, SV2B knockout mice reveal no overt pathology (Janz et al., 1999). It is suggested that one possible reason for this lack of consequence of loss of SV2B is that can be functionally replaced by SV2A, which appears to be co-expressed everywhere SV2B is normally expressed.
While the function of SV2A and other family members still remains unknown, the favored hypothesis is that this transporter homologue is a functional transporter for some common synaptic vesicle molecule. More specifically, there is evidence linking SV2A to the regulation of calcium-mediated vesicle exocytosis, and as a result, it is thought that it may be a Ca2+ transporter. SV2A and other family members may also have roles in the function of synaptic vesicles. Such roles may include modulating aspects of their formation, loading with neurotransmitter, fusion with the plasma membrane, re-cycling, and interactions with other proteins and cellular compartments and organelles. For instance it has been shown that SV2 proteins can interact with the synaptic vesicle protein synaptotagmin and the extracellular matrix protein laminin-1 (Carlson, Perspect. Dev. Neurobiol. 3(4):373–386 (1996)). The SV2 proteins may play important roles in regulating cytoplasmic or organellar calcium levels at the presynaptic terminal, and may also interact with N-type calcium channels on the plasma membrane, either directly or indirectly.